perf(community): optimize community content route payloads #1374

Context

Community pages are a compact MDX route family with their own persistent sidebar layout and independently assembled table of contents. They share content rendering primitives with documentation, but they should retain a route-local budget and browser journeys because their navigation, media, and loader data are different.

Build on the content highlighting and CSS ownership introduced by #1365 and #1367. Do not fold community-only resources back into the root or docs layout just because the pages resemble documentation.

Representative profiles

• /community/overview
• /community/content
• /community/roadmap

Use #1371 to establish the post-homepage production baseline.

Suggested PR stack

• Add community direct-load and client-navigation coverage, including desktop/mobile sidebar behavior, table-of-contents navigation, edit links, search activation, and a transition to and from the docs family.
• Profile loader serialization, sidebar-category construction, table-of-contents data, and rendered MDX. Centralize or precompute data only when it removes repeated route payload/markup without weakening deep-link behavior.
• Prerender static code presentation where it eliminates client work, while preserving content-boundary syntax registration and interactive Markdown components.
• Give community media accurate intrinsic dimensions and use responsive candidates only for meaningful raster placements; retain native elements and avoid a responsive-image runtime effect.
• Defer community-only interaction code until it is needed and consider React Compiler annotations solely for stable, measured leaf components.

Acceptance criteria

• A community direct load has no homepage, playground, package-index, or unrelated documentation payload beyond the intentional shared foundation/content boundary.
• Sidebar, table of contents, anchors, edit links, Markdown components, syntax highlighting, and search activation work through direct loads and client transitions without hydration or console errors.
• Every rendered image/video has accurate numeric dimensions and below-the-fold media is not promoted to an initial preload.
• Every completed layer reports the asset, request, DOM/media, and serialized-data delta against its immediate target.
• ReScript compilation, production build, focused Vitest/Cypress coverage, and the test(perf): establish post-homepage route-family performance baselines #1371 report pass. Visual snapshots remain unchanged unless explicitly approved.

Relates to #1354 and depends on #1371 for the common measurement contract.